There are three types of workpiece-gripping chucks available in the market today, and these are classified according to their method of opening and closing as hydraulic, pneumatic and mechanical. The hydraulic and pneumatic types are automatic, and the mechanical type is manually-operated. The hydraulic and pneumatic types are limited by the range of the workpiece diameter that may be mounted for machining with the selected jaw settings. If this range is exceeded, the lathe operation must be disrupted in order to manually change the jaw setting. The hydraulic type requires a hydraulic station and a power unit for each chuck, whereas the pneumatic type may use a central pneumatic station for all chucks used in the factory and a separate power unit for each chuck. The hydraulic chuck provides a higher maximum gripping force than a pneumatic chuck. The gripping force is changed by adjusting the pressure of the oil or air, respectively.
Mechanical chucks that are available in the market today are not automatic, and the operator must exert his own energy to open and close the chucks by means of an Allen wrench. Therefore the replacement of the workpiece takes longer than with the automatic types. Electromechanical power chucks have been developed. These chuck types are typically characterized as having a bulky construction.
Prior art power chuck apparatus has been disclosed in U.S. Pat. Nos. 4,951,535 and 4,758,006 to Hiestand. In the first patent, an adjustment member is used to actuate the clamping jaws of a chuck, with the adjustment member being rotationally driven by a motor and reduction gear mechanism not in-line with the spindle. In the second patent, an adjusting drive mechanism is provided for adjusting the clamping jaws, and is connected via a set of pulleys rotationally driven off the main spindle, again not in-line with it. This construction makes for a bulky arrangement, requiring much space. In one embodiment the adjusting drive mechanism is mounted within the lathe headstock, and the mechanism will consequently become contaminated by the presence of chips resulting from the machining process.
In PCT Patent application W094/14559 by Voisard, a scroll-type automatic chuck is disclosed using a vane motor operating under compressed air. The scroll-type chuck has a spiral crown which engages the teeth of the clamping jaws, so that rotation of the crown by means of the vane motor adjusts the jaw position. The construction uses a planetary gear which adds to the moment of inertia of the mechanism, making it heavier and less well-balanced. A high speed motor with a high torque is used. Since the preferred embodiment employs a gear ratio of 75, as specified in the application, high inertial forces are induced.
The aforementioned prior art power chucks do not take into account the additional gripping force that results from the inertial forces of the adjustment mechanism. Since the adjustment member rotates at a high velocity to achieve rapid jaw position adjustment, the abrupt change in velocity during which the workpiece is grasped within a split second produces deceleration of the adjustment member. Consequently high inertial forces are produced relative to the gripping force. Workpieces which require low gripping forces may be damaged if necessary precautions are not taken for the inertial forces, and it becomes impossible to achieve a low gripping force coupled with rapid jaw position adjustment.
The bulky construction associated with the prior art chuck designs makes them less desirable in many machine tool applications where space is at a premium. Therefore, it would be desirable to provide a compact, automatic power chuck with an adjustable gripping force, with rapid jaw position adjustment.
The workpiece diameter is an important measurement in automatic machining, since it determines whether the selected workpiece conforms to the machining program, and determines whether the workpiece was inserted in the correct position, i.e., whether the axis of the grasped workpiece is concentric and parallel to the axis of the lathe spindle. Currently, no accurate mechanism has been developed to measure the workpiece diameter and to provide feedback concerning the size and position of the workpiece. Information to confirm the correct workpiece and its position is beneficial to avoid costly damage to the cutting tool, the workpiece itself, and even to the lathe during machining steps.
Therefore, it would be desirable to provide information regarding the workpiece diameter measurement based on the operation of the automatic power chuck.